Demos: 3B-03 Paper & Cylinder

A single thickness of ordinary paper is wrapped tightly around a cylinder. A flame is played back and forth across the paper, resulting in an alternating pattern of charred and uncharred paper. It is revealed that the cylinder is a laminate consisting alternately of metal and wood. The paper is thin enough to conduct the heat without burning but if wood is immediately behind the paper, its poor conductivity allows the paper to reach its kindling temperature, whereas the metal conducts the heat away rapidly and keeps the kindling temperature from being reached.

Directions: If not already prepared, set up the demo prior to the lecture so that the students do not see how the cylinder is constructed. Cut off just enough paper to form a single layer around the cylinder. Wrap it tightly around the cylinder and secure it with a piece of tape. Hold the cylinder at one end (or place it on a stand) and play the flame from a torch or burner across the paper. Do this quickly and be prepared to blow out any flareups. After a bit, you should have a pattern similar to that shown above.

Suggestions for Presentation: Without comment, do the demonstration as outlined above. When you have obtained the pattern, ask the class if they can explain the pattern, i.e. how must the cylinder be constructed to achieve this pattern. Should someone guess that there are different materials behind each section, ask which would be the conductor and which would be the non-conductor. Surprisingly, many (if not most) students will guess that the charred part will have the conductor behind it. Why? Because metals heat up fast and so the paper is burned quickly. They are confusing specific heat with conductivity.

In a class a few years ago, a fairly insightful student guessed, after seeing the explanation, that if the cylinder were to be heated from the inside out, the patterns would be reversed! He was correct.